Superabsorbent polymers (SAPs) are materials that imbibe or absorb at least 10 times their own weight in aqueous fluid and that retain the imbibed or absorbed aqueous fluid under moderate pressure. The imbibed or absorbed aqueous fluid is taken into the molecular structure of the SAP rather then being contained in pores from which the fluid could be eliminated by squeezing. Some SAPs can absorb up to 1,000 times their weight in aqueous fluid.
One method of producing a SAP for use in agricultural applications involves graft polymerizing acrylonitrile onto a starch in the presence of an initiator, such as a ceric (+4) salt, to form a starch graft copolymer, and saponifying the nitrile groups with an alkali metal to form a saponificate having alkali carboxylate and carboxamide groups.
Saponification, however, may require expensive machinery and generates ammonia, which can be corrosive, costly to remove, and expensive to dispose of. Also, potassium hydroxide (KOH) added during saponification makes the saponified starch graft copolymer mixture basic. Acid, e.g., hydrochloric acid, nitric acid, sulfuric acid, or phosphoric acid, is added to the mixture in order to neutralize the pH of the starch graft copolymer mixture. If the amount of acid that must be added is significant, the absorbency of the SAP is reduced. The resulting waste solutions may also be expensive to dispose of because they include potassium and ammonium salts and other extraneous salts. Furthermore, acrylonitrile may be hazardous and expensive to dispose of.